Aiming at insulation resistance and make-break measurement of 32 core cable for operation monitor of the locomotive, using SCM control technique to control the ordering action of relay matrix, it realizes the automatic switching and measuring of core wire. In order to lower the power-waste and improve the system stability and measuring accuracy, we make use of the automatic controlling and switching of high-low pressure on voltage source, and automatically select the according sampling resistance which has a basis on the range of insulation resistance value.